The present invention relates to an optical recording/reproducing apparatus, and more particularly to a control operation at a time the digression of a light beam from a desired track (called "off-track") is caused by the abnormality in an optical disc or the above apparatus.
An optical disc apparatus has been put to practical use in which a recording film on a disc-shaped substrate is irradiated with a light beam to thermally form holes (called "pits") in the recording film, thereby recording data therein. In optical disc apparatuses of this type, concentric grooves or a spiral groove is generally formed on an optical disc as a recording medium to define a multiplicity of tracks. Usually, the tracks are arranged at intervals of about 1.6 .mu.m in the radial direction of the optical disc. Accordingly, even when the light beam digresses from a desired track only a little in the recording, a large amount of data will be destroyed. Therefore, it is necessary to accurately position the light beam on the track. That is, an optical disc apparatus is required to surely detect the digression of a light beam from a track (namely, to securely detect the off-track) in recording of data. An optical information processor provided with means for detecting the off-track has been disclosed in Maeda et al U.S. Pat. No. 4,554,652 issued Nov. 19, 1985 and owned by the present assignee. In this optical information processor of the Maeda et al patent, when the output of a detector for detecting the digression of the center of a light spot which is formed on an optical disc, from the center line of a track (hereinafter referred to as "tracking error signal") exceeds a predetermined level for more than a predetermined time, in a recording operation for writing data in the optical disc, it is judged that off-track is generated, and consequently the recording operation is stopped. At the same time, a controller is informed of the generation of off-track, and the same recording operation is performed for another sector or another region in the sector with off-track on the basis of a command from the controller. Usually, in an optical disc apparatus for recording digital data, after a data has been recorded in one track or in one sector of the track, a light head is returned back along the track, to check whether the recorded data can be properly read out. When a sector with a data being not normally read out is found, the data is again recorded in a spare sector namely, an alternate sector which is provided on the same track or a different track. Such operation is called "read-after-write operation". In a case where an off-track is detected from the tracking error signal during recording, if the recording operation is immediately stopped as mentioned above, an error in data recording will be detected in the read-after-write operation, and the data in the defective sector will be again written in an alternative sector. The off-track is caused mainly by a defect on the surface of an optical disc or a mechanical shock externally applied to an optical disc apparatus. In a case where the defect on the optical disc is nearly equal in size to a minimum defect capable of causing the off-track, owing to the difference in characteristics among optical disc apparatuses, a light spot follows a track in various manners when the light spot reaches the defect. That is, in an optical disc apparatus, the light spot may pass the defect without digressing from the track, though the off-track is detected at the defect. The same optical disc as used in the above optical disc apparatus can be loaded in another optical disc apparatus. In another optical disc apparatus, however, a light spot may digress from the track at the defect, and it may be impossible to write the same data as spoiled by the defect in an alternate sector and to read out the data from the alternate sector. The above-referenced Maeda et al U.S. patent does not teach a solution to the problem of offtrack due to the difference in characteristics among optical disc apparatuses.